Polyolefins are widely used commercially because of their robust physical properties. For example, various types of polyethylenes, including high density, low density, and linear low density polyethylenes, are some of the most commercially useful. Polyolefins are typically prepared with a catalyst that polymerizes olefin monomers.
Low density polyethylene is generally prepared at high pressure using free radical initiators or in gas phase processes using Ziegler-Natta or vanadium catalysts. Low density polyethylene typically has a density at about 0.916 g/cm3. Typical low density polyethylene produced using free radical initiators is known in the industry as “LDPE.” LDPE is also known as “branched” or “heterogeneously branched” polyethylene because of the relatively large number of long chain branches extending from the main polymer backbone. Polyethylene with a similar density that does not contain branching is known as “linear low density polyethylene” (“LLDPE”) and is typically produced with conventional Ziegler-Natta catalysts or with metallocene catalysts. “Linear” means that the polyethylene has few, if any, long chain branches and typically has a g'vis value of 0.97 or above, such as 0.98 or above. Polyethylenes having still greater density are the high density polyethylenes (“HDPEs”), e.g., polyethylenes having densities greater than 0.940 g/cm3 and are generally prepared with Ziegler-Natta or chrome catalysts. Very low density polyethylenes (“VLDPEs”) can be produced by a number of different processes yielding polyethylenes typically having a density 0.890 to 0.915 g/cm3.
Copolymers of polyolefins, such as polyethylene, have a comonomer, such as hexene, incorporated into the polyethylene backbone. These copolymers provide varying physical properties compared to polyethylene alone and are typically produced in a low pressure reactor, utilizing, for example, solution, slurry, or gas phase polymerization processes. Polymerization may take place in the presence of catalyst systems such as those employing a Ziegler-Natta catalyst, a chromium based catalyst, or a metallocene catalyst.
A copolymer composition, such as a resin, has a composition distribution, which refers to the distribution of comonomer that forms short chain branches along the copolymer backbone. When the amount of short chain branches varies among the copolymer molecules, the composition is said to have a “broad” composition distribution. When the amount of comonomer per 1000 carbons is similar among the copolymer molecules of different chain lengths, the composition distribution is said to be “narrow”.
The composition distribution influences the properties of a copolymer composition, for example, stiffness, toughness, environmental stress crack resistance, and heat sealing, among other properties. The composition distribution of a polyolefin composition may be readily measured by, for example, Temperature Rising Elution Fractionation (TREF) or Crystallization Analysis Fractionation (CRYSTAF).
A composition distribution of a copolymer composition is influenced by the identity of the catalyst(s) used to form the polyolefins of the composition. Ziegler-Natta catalysts and chromium based catalysts tend to produce compositions with broad composition distributions, whereas metallocene catalysts typically produce compositions with narrow composition distributions.
Furthermore, polyolefins, such as polyethylenes, which have high molecular weight, generally have desirable mechanical properties over their lower molecular weight counterparts. However, high molecular weight polyolefins can be difficult to process and can be costly to produce. Polyolefin compositions having a bimodal molecular weight distribution are desirable because they can combine the advantageous mechanical properties of a high molecular weight fraction of the composition with the improved processing properties of a low molecular weight fraction of the composition.
For example, useful bimodal polyolefin compositions include a first polyolefin having low molecular weight and high comonomer content (i.e., comonomer incorporated into the polyolefin backbone) while a second polyolefin has a high molecular weight and low comonomer content. As used herein, “low comonomer content” is defined as a polyolefin having 6 wt % or less of comonomer based upon the total weight of the polyolefin. The high molecular weight fraction produced by the second catalyst may have a high comonomer content. As used herein, “high comonomer content” is defined as a polyolefin having greater than 6 wt % of comonomer based upon the total weight of the polyolefin.
There are several methods for producing bimodal or broad molecular weight distribution polyolefins, e.g., melt blending, polymerizing in reactors in series or parallel configuration, or polymerizing in a single reactor with bimetallic catalysts. However, these methods, such as melt blending, are difficult to completely homogenize the polyolefin compositions and have high cost.
Furthermore, synthesizing these bimodal polyolefin compositions using a mixed catalyst system would involve a first catalyst to catalyze the polymerization of, for example, ethylene under substantially similar conditions as that of a second catalyst while not interfering with the catalysis of polymerization of the second catalyst.
There exists a need for catalyst systems that provide polyolefin compositions having novel combinations of comonomer content fractions and molecular weights. There is further a need for novel multi-catalyst systems where one catalyst does not inhibit the polymerization catalysis of any of the other catalysts or vice versa.
Catalysts for olefin polymerization are often based on cyclopentadienyl transition metal catalyst compounds as catalyst precursors combined with activators, typically an alumoxane or with an activator containing a non-coordinating anion. A typical metallocene catalyst system includes metallocene catalyst, activator, and optional support. Supported catalyst systems are used in many polymerization processes, often in slurry or gas phase polymerization processes.
For example U.S. Pat. No. 7,829,495 discloses Me2Si(fluorenyl)(3-nPr-Cp)ZrCl2 and U.S. Pat. No. 7,179,876 discloses supported (nPrCp)2HfMe2.
Further, Stadelhofer, J.; Weidlein, J.; Haaland, A. J. Organomet. Chem. 1975, 84, C1-C4 discloses preparation of potassium cyclopentadienide.
Additionally, Me2C(Cp)(Me3SiCH2-Ind)MCl2 and Me2C(Cp)(Me, Me3SiCH2-Ind)MCl2, where M is Zr or Hf have been synthesized and screened for the syndiospecific polymerization of propylene; see Leino, R., Gomez, F.; Cole, A.; Waymouth, R. Macromolecules 2001, 34, 2072-2082.
Metallocenes are often combined with other catalysts, or even other metallocenes, to attempt to modify polymer properties. See, for example, U.S. Pat. Nos. 8,088,867 and 5,516,848 (which discloses the use of two different cyclopentadienyl based transition metal catalyst compounds activated with alumoxane or non-coordinating anions). See also PCT/US2016/021748, filed Mar. 10, 2016, which discloses two metallocenes used to make ethylene copolymers.
Likewise, Me2C(Cp)(Me3SiCH2-Ind)MCl2 and Me2C(Cp)(Me, Me3SiCH2-Ind)MCl2, where M is Zr or Hf have been synthesized and screened for the syndiospecific polymerization of propylene; see Leino, R., Gomez, F.; Cole, A.; Waymouth, R. Macromolecules 2001, 34, 2072-2082.
Additional references of interest include: Hong et al. in Immobilized Me2Si(C5Me4)(N-t-Bu)TiCl2/(nBuCp)2ZrCl2 Hybrid Metallocene Catalyst System for the Production of Poly(ethylene-co-hexene) with Pseudo-bimodal Molecular Weight and Inverse Comonomer Distribution, (Polymer Engineering and Science-2007, DOI 10.1002/pen, pages 131-139, published online in Wiley InterScience (www.interscience.wiley.com) 2007 Society of Plastics Engineers); Kim, J. D. et al., J. Polym. Sci. Part A: Polym Chem., 38, 1427 (2000); Iedema, P. D. et al., Ind. Eng. Chem. Res., 43, 36 (2004); U.S. Pat. Nos. 4,701,432; 5,032,562; 5,077,255; 5,135,526; 5,183,867; 5,382,630; 5,382,631; 5,525,678; 6,069,213; 6,207,606; 6,656,866; 6,828,394; 6,964,937; 6,956,094; 6,964,937; 6,995,109; 7,041,617; 7,119,153; 7,129,302; 7,141,632; 7,172,987; 7,179,876; 7,192,902; 7,199,072; 7,199,073; 7,226,886; 7,285,608; 7,312,283; 7,355,058; 7,385,015; 7,396,888; 7,595,364; 7,619,047; 7,662,894; 7,829,495; 7,855,253; 8,110,518; 8,138,113; 8,268,944; 8,288,487; 8,329,834; 8,378,029; 8,575,284; 8,598,061; 8,680,218; 8,785,551; 8,815,357; 8,940,842; 8,957,168; 9,079,993; 9,163,098; 9,181,370; 9,303,099; U.S. Publication Nos. 2004/259722; 2006/275571; 2007/043176; 2010/331505; 2012/0130032; 2014/0031504; 2014/0127427; 2015/299352; 2016/0032027; 2016/075803; PCT Publication Nos. WO 97/35891; WO 98/49209; WO 00/12565; WO 2001/09200; WO 02/060957; WO 2004/046214; WO 2006/080817; WO 2007/067259; WO 2007/080365; WO 2009/146167; WO 2012/006272; WO 2012/158260; WO 2014/0242314; WO 2015/123168; WO 2016/172099; PCT Application No. PCT/US2016/021757, filed Mar. 10, 2016; EP 2 374 822; EP 2 003 166; EP 0,729,387; EP 0,676,418; EP 0 705 851; KR 20150058020; KR 101132180; Sheu, S., 2006, “Enhanced bimodal PE makes the impossible possible”, http://www.tappi.org/content/06asiaplace/pdfs-english/enhanced.pdf; and Chen et al., “Modeling and Simulation of Borstar Bimodal Polyethylene Process Based on Rigorous PC-SAFT Equation of State Model”, Industrial & Engineering Chemical Research, 53, pp. 19905-19915, (2014). Other references of interest include: U.S. Publication No. 2015/0322184 and A. Calhoun, et al. “Polymer Chemistry”, Chapter 5, pages 77-87.
There is still a need in the art for new and improved catalyst systems for the polymerization of olefins, in order to achieve increased activity or enhanced polymer properties, to increase conversion or comonomer incorporation, or to alter comonomer distribution. There is also a need for supported catalyst systems and processes for the polymerization of olefins (such as ethylene) using such catalyst systems to provide ethylene polymers having the unique properties of high stiffness, high toughness and good process ability.